From Laplace to Langlands via representations of orthogonal groups

Transcription

1 From Laplace to Langlands via representations of orthogonal groups Benedict H. Gross Department of Mathematics, Harvard University Cambridge, MA Mark Reeder Department of Mathematics, Boston College Chestnut Hill, MA August 31, Introduction In the late 1960s, Robert Langlands proposed a new and far-reaching connection between the representation theory of Lie groups over real and p-adic fields, and the structure of the Galois groups of these fields [24]. Even though this local Langlands correspondence remains largely conjectural, the relation that it predicts between representation theory and number theory has profoundly changed our views of both fields. Moreover, we now know enough about the correspondence to address, and sometimes solve, traditional problems in representation theory that were previously inaccessible. Roughly speaking, the local Langlands correspondence predicts that complex irreducible representations of a reductive group G over a local field k should be Supported by NSF grant DMS Supported by NSF grant DMS

2 parametrized by certain homomorphisms of the Galois group (or more generally the Weil group) of k into a complex Lie group L G which is dual to G, in a sense that will be explained below. In this article, our aim is two-fold: First, we want to introduce the Langlands correspondence for reductive groups over local fields, giving many examples along the way. Second, we want to show how the Langlands correspondence, combined with some ideas from number theory, can be used to study the classical problem of restricting irreducible discrete series representations from SO 2n+1 to SO 2n. The conjectures of Gross-Prasad [14] describe these restrictions in terms of symplectic local root numbers attached to the Langlands parameters of representations of orthogonal groups. These root numbers are number-theoretic invariants of the parameters, but to test the conjectures one needs to construct the corresponding irreducible representations. This has been done recently, for some interesting discrete series parameters, in [9] and [22]. Specializing this constuction to orthogonal groups, we verify the Gross-Prasad conjectures for these parameters. Since the restriction problem we consider has roots in the very origins of representation theory, we have tried to place our results on orthogonal groups in an historical context. Moreover, as representation theory is now used in many areas of mathematics, we have also tried to make much of this paper accessible to a wide audience, by raising the prerequisites as gradually as possible. This aim has forced us to omit much recent work on the local Langlands correspondence, such as [18], [20] for GL n, and [26], [27] on unipotent representations. We begin with a discussion of spherical harmonics for the compact real group SO(3) of rotations in R 3. We will see that each irreducible representation of SO(3) is naturally paired with a discrete series representation of the noncompact orthogonal group SO(2, 1). This is our first example of an L-packet of representations attached to a Langlands parameter. We then introduce the p-adic versions of SO(3) and SO(2, 1), and arrive at analogous L-packets of discrete series representations. In both cases, we discuss the restriction of the representations to the subgroup SO(2). Next, we return to real groups, in a more general setting. After a review of the fundamental results on discrete series for real Lie groups, due to Harish-Chandra and Schmid, we partition their representations into L-packets, with illustrations in the orthogonal case. This permits us to formulate the Gross-Prasad conjecture on restriction from SO 2n+1 to SO 2n, which in the real case generalizes classical branching laws. In section 6 we turn to a general description of Langlands parameters, intro- 2

3 ducing the Weil group of a local field and the complex Lie group L G mentioned above. Here the prerequisites increase; to help the reader we give some background on root data and quasi-split groups. We then show how the L-packets constructed by Harish-Chandra and Schmid correspond to real Langlands parameters, and we illustrate these parameters for orthogonal groups in more detail. The remainder of the paper is devoted to the p-adic case, where much of the Langlands correspondence remains conjectural. We focus on parameters which we call tame regular discrete, and which are analogous to discrete series parameters in the real case. In section 11 we briefly outline the recent construction of the L-packets of irreducible representations associated to these parameters. After a few words about Bruhat-Tits theory, we then give more details about this construction, in section 13. We then turn to the parameters for p-adic orthogonal groups, and introduce local epsilon factors from number theory, in order to state the Gross-Prasad conjecture on restriction from SO 2n+1 to SO 2n in the p-adic setting. The rest of the paper is devoted to verifying this conjecture, for tame regular discrete Langlands parameters. The proof invokes a result in [28] on the restriction of Deligne-Lusztig characters for finite orthogonal groups. There is no denying that the Langlands correspondence throws many technical barriers in the way of the interested mathematician, neophyte and expert alike! We hope that the mixture of general theory with explicit examples will enable many readers to hurdle, or at least to see over these barriers, in order to appreciate this fruitful interaction between representation theory and number theory. We thank J.-P. Serre and N. Wallach, for helpful comments on an earlier version of this paper. In addition, the referees made numerous insightful criticisms which, we believe, enabled us to improve the exposition. Contents 1 Introduction 1 2 Spherical Harmonics 5 3 p-adic SO(3) 6 4 Discrete series for real groups 8 3

4 5 Discrete series for real orthogonal groups and branching laws 15 6 Introduction to Langlands parameters Weil groups Quasi-split groups and the L-group Langlands parameters Real orthogonal groups again 26 8 Pure inner forms of p-adic groups 28 9 The p-adic Langlands conjecture Parameters for some p-adic discrete series Tame regular discrete parameters Anisotropic tori Summary Tame regular discrete series L-packets for p-adic groups: introduction Bruhat-Tits theory The building The geometry of an apartment and pure inner forms Parahoric subgroups Tame regular discrete series L-packets for p-adic groups: completion L-packets for p-adic orthogonal groups L-packets for odd p-adic orthogonal groups L-packets for even p-adic orthogonal groups Symplectic root numbers Restriction and symplectic root numbers Calculation of the branching character The representations π and σ of SO 2n+1 and SO 2n 54 4

5 19 The multiplicity of σ in the restriction of π Restriction of Deligne-Lusztig representations 56 2 Spherical Harmonics One of the earliest results in harmonic analysis for a nonabelian group was the decomposition of square-integrable functions on the sphere S 2 : L 2 (S 2 ) = ˆ π m (Hilbert direct sum) (1) m 0 into eigenspaces of the spherical Laplacian. The eigenspace denoted π m has dimension 2m + 1, and has associated eigenvalue λ = m(m + 1). It affords an irreducible representation of the spherical rotation group SO(3). Every irreducible complex representation of the compact group G = SO(3) is isomorphic to some π m. Let H be the subgroup of G fixing a point on S 2. Then H is isomorphic to the rotation group in the plane orthogonal to the line through the fixed-point and its antipode. Thus, we may identify H = SO(2) = {z C : z = 1}. For k Z, let χ k : H C be the unitary character defined by χ k (z) = z k. Then the restriction of the representation π m from G to H decomposes as a direct sum of irreducible representations: Res H (π m ) = χ k. (2) k m For all of the above see, for example, [43, III.9] The decompositions (1) and (2) were used by theoretical physicists to model energy levels of the hydrogen atom [45]. Physicists also initiated the study of representations of the non-compact form G = SO(2, 1) of G; see [1]. This is the group of orientation-preserving isometries of the quadratic form Q = x 2 +y 2 z 2, and is isomorphic to PGL 2 (R). The irreducible complex representations of a Lie group G which occur as closed subspaces of L 2 (G) are called discrete series. If G is compact then every irreducible representation is in the discrete series. 5

6 For G = SO(2, 1) the discrete series are representations π m, parametrized by integers m 0. The subgroup H G fixing a vector in R 3 whose orthogonal complement is definite is isomorphic to SO(2). The representation π m can be characterized by its restriction to SO(2): Res H (π m ) = ˆ χ k. (3) k >m To summarize, for each integer m 0 we have a set Π m = {π m,π m} (4) of irreducible representations of G = SO(3) and G = SO(2, 1). Every irreducible representation χ k of SO(2) occurs once in the restriction of either π m or π m for every m, but not in both. We will see that this appears to be a general phenomenon for L-packets (of which Π m is an example) of representations of orthogonal groups over real and p-adic fields. 3 p-adic SO(3) A similar result holds for representations of the rotation group SO(V ), for 3- dimensional orthogonal spaces V over p-adic fields. Let k = Q p be the field of p-adic numbers, let o = Z p be the ring of integers in k, and let f = o/po Z/pZ be the residue field. Let k 2 be the unramified quadratic extension of k, with ring of integers o 2, and residue field f 2 = o 2 /po 2, with p 2 elements. If p > 2 then k 2 = k( u) and o 2 = o[ u] for u o a unit which is not a square. Let τ be the nontrivial automorphism of k 2 which is trivial on k; on o 2 we have the formula τ(a) a p mod po 2. We begin with two-dimensional spaces which are analogous to the complex plane. The space W = k 2 is an orthogonal space over k of rank two, whose quadratic form a a τ(a) is the norm from k 2 to k. The rotation group of W is H = SO(W) = {a k 2 : τ(a) = a 1 } = {a o 2 : τ(a) = a 1 }. There is extra structure that did not appear in the real case: we also have a quadratic f-space W = f 2 with quadratic form α α p+1. Reduction modulo p from o 2 to f 2 gives a surjective group homomorphism H H = SO( W) = {α f 2 : α p = α 1 }. 6

7 The group H is cyclic of order p + 1. We say a character χ of H is tame if it factors through H, and is regular if χ 2 1. In three dimensions, there are exactly two distinct orthogonal spaces of rank three over k, up to equivalence and scaling [32, Chap. IV]. We denote them by (V,Q) and (V,Q ). They both contain W and have orthogonal decompositions V = W e, Q(e) = p, V = W e, Q (e ) = 1. In the first case, Q(e) = p is not a norm from k 2, Q does not represent zero nontrivially on V, and G = SO(V ) is compact. In the second case, Q (e ) = 1 is a norm from k 2, Q has nontrivial zeros on V, and G = SO(V ) is noncompact. The group H = SO(W) is a subgroup of both G and G, fixing the vectors e and e, respectively. We will now construct irreducible representations π χ and π χ of G and G, starting with tame regular characters χ of H. The group G preserves the lattice L = o 2 oe in V, and the form induced by Q on L/pL has one dimensional radical. This implies a surjection G O( W). Hence G has a normal subgroup K of index two, which surjects onto H = SO( W). We view χ as a character of K, and define π χ as the induced representation: π χ = Ind G K χ. (5) More precisely, induced representations are defined as follows. If K is a finiteindex subgroup of a group G and ρ : K GL(V ρ ) is a representation of K, then Ind G K ρ = {f : G V ρ such that f(kg) = ρ(k)f(g), for all k K, g G}, (6) and the group G acts on Ind G K ρ by (g f)(x) = f(xg) for x,g G. We have dim Ind G K ρ = [G : K] dim(v ρ ). We will use various elementary properties of induced representations without further comment. See, for example, [34, chap. 7]. Returning to π χ, we have dim(π χ ) = 2. Since χ is conjugate to χ 1 in O(W) G, it follows that π χ is isomorphic to π χ 1. Restricting π χ to H, we find Res H (π χ ) = χ χ 1. (7). 7

8 To define π χ, it is convenient to view the noncompact group G as PGL 2 (k), whose Lie algebra is a three-dimensional quadratic space isometric to (V,Q ). This group has a maximal compact subgroup K = PGL 2 (o) G. In turn, K maps surjectively onto Ḡ = PGL 2 (f). The latter is a finite group of order p(p 2 1). The group H embeds as a subgroup of Ḡ, and a cohomological form of induction, due to Drinfeld in this case and extended by Deligne and Lusztig [10], associates to the data ( H,χ) an irreducible representation R(χ) of Ḡ, of dimension p 1. For the particular group Ḡ = PGL 2 (f), the character of the representation R(χ) was discovered by Frobenius [11, section 9] in one of his first papers on representation theory. The irreducible representation R(χ) of Ḡ is characterized by its restriction to H: Res H (R(χ)) = η. η Irr( H) η χ,χ 1 We pull R(χ) back to a representation of K via the homomorphism K PGL 2 (f), and define π χ = Ind G K R(χ). (8) The group K has infinite index in G. Induction is defined as in (6), but now we require the functions f Ind G K R(χ) to have compact support, and to be invariant under a compact open subgroup of G. The representation π χ is irreducible and belongs to the discrete series of G. All characters of H appear in the restriction Res H (π χ) with multiplicity one, except for χ and χ 1. See, for example, [41]. In summary, as in the real case, the representations π χ of G and π χ of G, defined in (5) and (8), form an L-packet Π χ = {π χ,π χ} with the property that every character of H appears with multiplicity one in π χ or π χ, but not both. 4 Discrete series for real groups It would now be helpful if the reader had some familiarity with the basic structure theory of semisimple Lie groups (cf. [42]). It may also help to glance at the next 8

9 section, where the general ideas below are illustrated for orthogonal groups. A real semisimple Lie group G has discrete series representations precisely when it contains a compact maximal torus. The discrete series of such groups were constructed in a uniform way, first by Harish-Chandra [17] on the level of characters, later by Schmid [30] on the level of vector spaces. There are many excellent treatments of discrete series for real groups in the literature, for example [31]. The following sketch has the advantage of showing how discrete series are naturally grouped into L-packets containing representations of different inner forms of a compact Lie group. We also avoid certain technicalities by restricting the class of Lie groups under discussion. Our treatment applies to all real orthogonal groups that have discrete series, like SO(2, 1) = PGL 2 (R), as well as to all semisimple Lie groups with trivial center, but it omits other Lie groups with discrete series, like SL(2, R). The Lie groups we consider are those obtained from compact Lie groups G by twisting inside the complexification G C, as will be explained below. Let G be a compact connected Lie group with finite center. Up to conjugacy, G contains a unique maximal torus S (S 1 ) n, a direct product of copies of the circle group S 1. The number n is called the rank of G. Associated to G is its complexification G C which is a connected complex Lie group containing G as a maximal compact subgroup. For example, if G = SO(2n+1) is the rotation group of a positive definite quadratic form Q on R 2n+1, then G C = SO(2n + 1, C) is the subgroup of SL(2n + 1, C) preserving the extension of Q to C 2n+1. We need to recall some structure theory of complex Lie groups. The group G C contains the complexification S C (C ) n of S. We choose a Borel subgroup (i.e., a maximal connected solvable subgroup) of G C containing S C. Borel subgroups can also be characterized as follows. There is a unique (up to isomorphism) complex projective variety B of maximal dimension, on which G C acts transitively by holomorphic maps. The variety B is called the flag variety of G C. The Borel subgroups in G C are the stabilizers of points in the flag variety B. In fact, Borel subgroups are their own normalizers, so we can think of B as the variety of Borel subgroups of G C, where G C acts by conjugation. The complex torus S C has only finitely many fixed points in B. By choosing B C to contain S C, we are choosing B C B to be one of these S C -fixed points. We can then identify B = G C /B C. The flag varieties of classical groups are familiar objects from algebraic geometry. For example, a Borel subgroup in SO(2n + 1, C) is the stabilizer of an 9

10 isotropic flag in C 2n+1. The latter are nested chains of subspaces 0 = V 0 V 1 V n C 2n+1, with dim V i = i, on which the quadratic form Q vanishes identically. In particular, the flag variety of SO(3, C) is the quadric in CP 2 defined by Q, and is isomorphic to CP 1. We now recall a bit more structure theory. The group S C acts on the Lie algebras g and b of G C and B C, respectively. Hence S C acts on the quotient g/b. The positive roots of S C are the characters of S C appearing in its action on g/b. We let Φ + denote the set of positive roots. Within Φ + there is a unique subset of simple roots, with the property that every root in Φ + can be written uniquely as a non-negative integral combination of roots in. Each positive root appears in g/b with multiplicity one. Since g/b is the tangent space to B at B C, we have dim B = Φ +. The negative roots Φ are those nontrivial characters of S C appearing in b (this seemingly odd convention is standard in the theory). We have not forgotten the compact group G that we started with. There is an analogue of complex conjugation, which is an automorphism of order two of the abstract group G C, denoted g ḡ. This automorphism preserves S C, and we have G = {g G C : ḡ = g}, S = {s S C : s = s}. Since the automorphism g ḡ preserves S C, it permutes the roots. In fact, it interchanges Φ + and Φ. We will twist this automorphism to obtain various non-compact real groups inside G C, as follows. Let S 2 = {s S : s 2 = 1} be the 2-torsion subgroup of S. Then S 2 {±1} n is an elementary abelian two-group of rank n. For each s S 2, we define a subgroup G s G C by G s := {g G C : ḡ = sgs}. (Recall that s = s 1.) In other words, G s consists of the fixed-points of the nontrivial involution g sḡs. The group G s is a real Lie group, which is usually 10

11 non-compact. The centralizer K s of s in G is a maximal compact subgroup of G s, and S K s G s. (9) When s = 1, we have K s = G s = G. Since the automorphism g ḡ interchanges Φ + and Φ, it follows that G s B C = S, for any s S 2. Therefore, the homogeneous space G s /S can be identified with the G s -orbit of B C in B. This orbit is open in B, so the manifold G s /S acquires a complex structure, via its identification with the domain D s = G s B C /B C G C /B C = B. The homogenous space K s /S also has a complex structure; it is the flag variety of the complexification of K s. In fact, K s /S is a maximal compact complex submanifold of the domain D s = G s /S. The fibration K s /S D s = G s /S G s /K s of real manifolds shows that D s can be thought of as the family of flag varieties of maximal compact subgroups of G s. For G = SO(3), we have D 1 = B = CP 1 and D 1 = CP 1 RP 1 is the union of two copies of the unit disk. The existence and parametrization of the discrete series was obtained by Harish- Chandra, in terms of their characters. The vector space realization of the discrete series, in terms of cohomology of line bundles on the domain D s, is due to Schmid, and was in part conjectured by Langlands. We now sketch Schmid s construction. Our line bundles will be restrictions to D s of G C -equivariant holomorphic line bundles on B. The latter correspond bijectively to (algebraic) characters of B C, or even to characters of S C, because B C is the semidirect product B C = S C U C of S C by the commutator subgroup U C of B C, so we may identify the characters of B C and S C. Given a character χ : S C C, we pull χ back to B C and define L(χ) = G C BC C χ, which is the quotient of G C C by the B C -action: b (g,z) = (gb 1,χ(b)z). Projection onto the left factor makes L(χ) a holomorphic line bundle over B on which G C acts by holomorphic vector bundle maps. 11

12 Written additively, the character group X(S C ) of S C is isomorphic to Z n ; to (χ 1,...,χ n ) Z n corresponds the character χ : S C C given by χ(z 1,...,z n ) = n k=1 z χ k k. So the line bundles L(χ) constructed above are parametrized by points χ in the lattice X(S C ). It turns out to be helpful to shift this lattice by the element ρ = 1 2 α Φ + α. Note that ρ does not necessarily belong to X(S C ), but 2ρ, being a sum of roots, does belong to X(S C ). Since 2ρ is the sum of the characters of S C in the tangent space g/b to B at B C, it follows that L( 2ρ) is the canonical bundle of B, whose sections are the top-dimensional holomorphic differential forms on B. We are going to construct representations of G s parametrized by those elements of the shifted lattice ρ + X(S C ) which satisfy a certain positivity property. We call elements of ρ + X(S C ) characters although they may not be characters of S C, but rather of a two-fold cover of S C. To state the positivity property, we must first recall the notion of co-root. For each root α Φ +, there is a homomorphism ψ α : SL 2 (C) G C with differential ([ ]) dψ α, sending the diagonal matrices of SL 2 (C) into S C and such 0 1 that dψ α g projects to a non-zero vector in the α-eigenspace of S 0 0 C in g/b. The 1-parameter subgroup ([ ]) z 0 ˇα : C S C, z ψ α 0 z 1 is called the co-root of α. Each co-root ˇα defines a homomorphism, ˇα : X(S C ) Z such that χ (ˇα(z)) = z χ,ˇα. The character ρ has the property that 2ρ, ˇα = 2 (10) 12

13 for all simple roots α. It follows that χ, ˇα Z for every character χ ρ + X(S C ). We say that χ ρ + X(S C ) is positive if χ, ˇα 1 for all ˇα Φ +. By (10), the character χ = ρ is positive, and is the least positive of all positive characters in ρ + X(S C ). To every positive character χ ρ + X(S C ), we will associate 2 n irreducible discrete series representations π(χ,s), indexed by the elements s S 2. Take s S 2, giving rise, as above, to a domain D s B, with D s G s /S. Since G s acts on D s and on the restriction of the equivariant line bundles L( ), the group G s acts on the holomorphic sheaf cohomology groups H i (D s, L( χ ρ)). Schmid proves that H i (D s, L( χ ρ)) is nonzero only in degree i = q(s), where q(s) is the complex dimension of K s /S. This nonvanishing cohomology group π(χ,s) := H q(s) (D s, L( χ ρ)) is a Fréchet space affording an irreducible representation π(χ,s) of G s, which Schmid proves is in the discrete series (he shows that the canonical map from L 2 -cohomology to sheaf cohomology is injective with dense image). When s = 1, the group G = G s = K s is compact, and π(χ,s) is the finite dimensional representation of G with highest weight χ ρ for S, by the Borel- Weil-Bott theorem. In general, the cohomological restriction map H q(s) (G s /S, L( χ ρ)) H q(s) (K s /S, L( χ ρ)) is surjective, so that the irreducible K s -representation H q(s) (K s /S, L( χ ρ)), of highest weight χ ρ s appears in π(χ,s), where ρ s = 1 α(s)α. (11) 2 α Φ + (Note that α(s) = ±1 for any α Φ +, since s 2 = 1.) Schmid proves, moreover, that π(χ,s) is the unique irreducible representation of G s containing the K s -representation of highest weight χ ρ s with multiplicity one and no other K s - representations of the form χ ρ s +A, where A is a non-empty sum of roots in Φ + which are trivial on s. Thus, π(χ, s) is completely characterized by its restriction to the maximal compact subgroup K s of G s. 13

14 To summarize: For every positive character χ ρ + X(S C ), we have constructed 2 n discrete series representations π(χ,s), one for each group G s, with s S 2. We call the set of these representations Π(χ) := {π(χ,s) : s S 2 } an L-packet. We still have a few loose ends to tie up. The first point is that many of the groups G s will be isomorphic to one another, so we are really getting different representations of the same group. This is because of the action of the Weyl group W := N G (S)/S, the quotient of the normalizer of S in G by S. The Weyl group acts on S by conjugation (since S is abelian), preserving the finite subgroup S 2. If s is in the W -orbit of s in S 2, then s = s n = n 1 sn for some n N G (T). We find that G s = n 1 G s n in G C and K s = n 1 K s n in G. Hence conjugation by n gives isomorphisms S K s G s S K s G s. (12) The choice of n such that s = s n is not unique. However, two choices of n differ by an element of N G (S) K s. This implies that if π : G s GL(V ) is a representation of G s on a Hilbert space V, then the isomorphism class of the representation π n : G s GL(V ) given by π n (g) = π(ngn 1 ) is independent of the choice of n. Hence, we have two representations π(χ,s) and π(χ,s ) n of G s. It turns out that these two representations of G s are isomorphic if and only if s = s. Thus, we have constructed exactly W s distinct representations of G s. If we choose representatives s 1,...s m of the W -orbits in S 2, then our L-packet Π(χ) is partitioned as m Π(χ) = Π(χ,p), (13) p=1 where Π(χ,p) consists of [W : W sp ] distinct representations of G sp. There is one more subtle point: it could even happen that G s G s with s,s belonging to different W -orbits in S 2. In fact, G s G s if and only if there is w W such that s w = zs, for some z in the center of G. Thus, it can happen 14

15 that the same representation of the same group appears more than once in our L- packet Π(χ). This phenomenon cannot happen for odd orthogonal groups, since these have trivial center. However, it does happen for even orthogonal groups, as will be illustrated in the next section. 5 Discrete series for real orthogonal groups and branching laws We now illustrate the previous general theory of discrete series in the case of real orthogonal groups in higher dimensions. Once again, we begin with spherical harmonics. Assume that d 2. The square-integrable functions on the sphere S d behave like (1): there is a decomposition L 2 (S d ) = ˆ π m, (14) into eigenspaces of the Laplacian, where π m is the eigenspace with eigenvalue m(m+d 1). The space π m is the restriction to S d of the harmonic polynomials of degree m on R d+1 ; it affords an irreducible representation of the rotation group SO(d + 1), of dimension ( ) ( ) d + m d + m 2 dim π m =. d d For all of the above, see, for example, [21, p.17, Thm. 3.1]. For d 3, not all irreducible representations of SO(d + 1) appear in L 2 (S d ). A representation π appears in L 2 (S d ) precisely when π contains a nonzero vector invariant under SO(d). Equivalently, the representations π m occurring in (14) are precisely the irreducible representations of SO(d + 1) whose restriction to SO(d) contains the trivial representation. The decomposition of an irreducible representation of SO(d + 1) when restricted to SO(d) is known classically as a branching law. Here we will generalize branching laws to the restriction of discrete series representations of both compact and non-compact orthogonal groups. We begin by describing the discrete series parameters for the group G = SO(2n + 1). Here S = SO(2) n (S 1 ) n, and the character group of S C = (C ) n is n X(S C ) = Ze k, 15 m 0 k=1

16 where e k : S C C is projection onto the k th factor of S C. The Weyl group W n is the group of the n-cube, of order 2 n n!. Viewed as a group of permutations of X(S C ), the group W n is the semidirect product of the symmetric group S n of permutations of the e i, with the normal subgroup {±1} n of sign changes in the e i. We may choose our Borel subgroup B C so that and Φ + = {e i ± e j : 1 i < j n} {e k : 1 k n}, (15) ρ = 1 α = 2 α Φ + n (n k + 1)e 2 k = (n 1,n 3,..., 1) k=1 The positive characters χ ρ + X(S C ) are given by χ = χ k e k with each χ k Z and χ 1 > χ 2 > > χ n > 0. We have S 2 = {±1} n SO(2) n, and the W n -orbit of s S 2 is determined by the number of components k such that e k (s) = 1. If s S 2 has p such components equal to 1 and p + q = n then G s SO(2p, 2q + 1) is the special orthogonal group of a real quadratic form with signature (2p, 2q+1). The stabilizer of s in W n is W p W q, so the W -orbit of s has size ( n p). This is the number of distinct discrete series representations of G s in the L-packet Π(χ) = Π G (χ). For the L-packet {π m,π m} defined in (4) for n = 1, we have χ = (m + 1)e 2 1. The line bundle L m = L( χ ρ) on CP 1 is the one traditionally denoted by O( 2 2m). The representations π m, π m are realized on the cohomology groups π m = H 1 (CP 1, L m ), and π m = H 0 (CP 1 RP 1, L m ). The group H = SO(2n) contains the same maximal torus as SO(2n + 1), so we again have X(S C ) = n k=1 Ze k. The Weyl group W is the subgroup of index two in W n, generated by S n and an even number of sign changes. We may choose our Borel subgroup B C so that Φ + = {e i ± e j : 1 i < j n}, (16) 16

17 and ρ = n (n k)e k = (n 1,n 2,...,1, 0). k=1 The positive characters η ρ + X(S C ) are those of the form η = k η ke k with η k Z and η 1 > η 2 > > η n 1 > η n. If t S 2 has p components k such that e k (t) = 1 and p + q = n then H t SO(2p, 2q). For each positive character η for H, the construction of the previous section gives us an irreducible discrete series representation of H t, which we will denote by σ(η,t), so as not to confuse it with the analogous representation π(χ,s) of G s. We then have a discrete series L-packet Π H (η) = {σ(η,t) : t S 2 }. The W -orbit of t again has size ( n p). Since SO(2p, 2q) SO(2q, 2p), we see that there are two W -orbits in S 2 giving rise to the isomorphic groups H t H t, when p q. These discrete series representations occur twice in Π H (η), once for H t SO(2p, 2q), and once for H t SO(2q, 2p). For the one-dimensional representation χ m of SO(2), we have η = me 1. The L-packet Π H (η) consists of the two occurrences of χ m, once for SO(2, 0) and once for SO(0, 2). Let χ and η be positive characters for G and H respectively, giving rise to L-packets Π G (χ) and Π H (η), as above. In [14], a branching law was proposed for the multiplicity of representations in Π H (η) in the restriction of representations in Π G (χ). According to this conjecture, the characters χ = χ k e k and η = η k e k determine elements s,t S 2 by the conditions e k (s) = ( 1) ν(k), e k (t) = ( 1) µ(k), (17) where ν(k) is the total number of i,j such that χ i,η j χ k, and µ(k) is the total number of i,j such that χ i,η j η k. The coordinates of s and t have the same number p of coordinates equal to 1, so and π := π(χ,s) is a representation of G s SO(2p, 2q + 1) σ := σ(η,t) is a representation of H t SO(2p, 2q). The prediction is that σ occurs in the restriction of π, with multiplicity one, i.e., that dim Hom Ht (π,σ) = 1, (18) 17

18 and that no other representation in the L-packet Π H (η) should appear in the restriction of a representation in the L-packet Π G (χ). Formula (18) was proved in [15], in the cases where the restriction of π to SO(2p, 2q) decomposes discretely as a Hilbert direct sum. (In most cases, the restriction will have continuous spectrum.) This discrete decomposition occurs when χ = χ k e k and η = η k e k interlace in the following way: For some integer 0 p n we have η 1 > χ 1 > > η p > χ p, χ p+1 > η p+1 > > χ n > η n. (19) Then s = t and we have e k (s) = e k (t) = { 1 if 1 k p +1 if p < k n. The representations π = π(χ,s) and σ = σ(η,s) are small discrete series: their restriction to the compact subgroup SO(2p) H s G s contains each irreducible representation of SO(2p) with finite multiplicity. For example, if χ 1 > η 1 > χ 2 > η 2 > > χ n > η n, (20) then p = 0, and s = t = 1. In this case, G = SO(2n + 1) and H = SO(2n) are compact, and we recover a classical branching law (see [2]) for the finite dimensional representation π(χ, 1): Res H (π(χ, 1)) η σ(η, 1), where we sum over all positive characters η for SO(2n) satisfying (20). The trivial representation of SO(2n) is π(ρ, 1), where we recall that ρ = ρ H = (n k)e k for H = SO(2n). For η k = n k, the characters χ = χ k e k satisfying (20) are those of the form χ = me 1 + ρ, m 0, where we recall that ρ = ρ G = n k=1 (n k+ 1 2 )e k for G = SO(2n+1). The representation π(me 1 +ρ, 1) is the representation π m appearing in the decomposition (14) of functions on the sphere S d. 18

19 At the other extreme, if η 1 > χ 1 > η 2 > χ 2 > > η n > χ n, (21) we have p = n, G s = SO(2n, 1) and H s = SO(2n) has index two in the maximal compact subgroup K s = O(2n) of G s. We recover another known branching law, which is a special case of Blattner s formula, proved by Hecht and Schmid [19]: Res Hs (π(χ,s)) = ˆ σ(η,s), a Hilbert direct sum over all η = η k e k satisfying (21). η 6 Introduction to Langlands parameters All of the results on branching laws for orthogonal groups which we have discussed so far fit nicely into a general conjecture [14], using the language of Langlands parameters for irreducible representations of reductive groups G over local fields k. The rough idea is that irreducible representations of these groups should be parametrized by homomorphisms from the Weil group of k into a group L G which is dual to G and encodes the arithmetic structure of G as a group over k. Then properties of irreducible representations, such as branching laws, should be determined by number-theoretic invariants of the corresponding parameters. Historically, and in this paper so far, this connection between number theory and representation theory was not evident in the real case, because the Galois theory of R is rather simple. However, it is enlightening to rephrase the above results on L-packets for real groups in terms of the Weil group of R. For nonarchimedean fields k, the Weil group is essential for describing L-packets and branching laws. In this chapter we define Weil groups for real and p-adic local fields, and give an introduction to some of the local number theory that is needed to construct p-adic L-packets. This is followed by some background on quasi-split groups, which is preliminary to the definition of the group L G. With these ingredients in hand, we give a preliminary definition of a Langlands parameter, and then consider some refinements of this definition. 19

20 6.1 Weil groups Let k be a local field of characteristic zero. That is, k is either R, C or a finite extension of Q p. Fix an algebraic closure k of k. The Weil group W(k) of k is a locally compact group, which comes with a group homomorphism with dense image, and an isomorphism W(k) Gal( k/k) W(k) ab k. Here W(k) ab is the quotient of W(k) by the closure of its commutator subgroup. Weil groups can be described concretely. For archimedean k they are given by: W(C) = C, W(R) = N H (C ). (22) That is, W(R) is the normalizer of C in H, the multiplicative group of Hamilton s quaternions. The group W(R) contains the normal subgroup C with index two; the nontrivial coset jc is represented by j H where j 2 = 1 and jzj 1 = z for all z C. Thus, we have an exact sequence 1 C W(R) Gal(C/R) 1 (23) with j mapping to complex conjugation. This sequence is not split; since H is a division algebra, the only involutions in H are ±1. If k is non-archimedean, then the description of W(k) is a bit more involved. Roughly speaking, W(k) is the Galois group of k/k with the topology relaxed, so as to allow more continuous representations. More precisely, if k has residue field f of order q, then we have an exact sequence 1 I Gal( k/k) Gal( f/f) 1, whose kernel I is called the inertia group of k. Any element F Gal( k/k) whose image in Gal( f/f) is the automorphism α α q of f is called a Frobenius element. We fix a choice of F once and for all. The image of F in Gal( f/f) generates a dense subgroup, isomorphic to Z. 20

21 The Weil group of W(k) is defined to be the subgroup of Gal( k/k) generated by F and the inertia subgroup I. Thus, W(k) = n Z IF n = I F, and we have an exact sequence (analogous to (23)) 1 I W(k) Z 1. (24) A subset of W(k) is open iff its intersection with every coset of I is open, where I has the profinite topology inherited from Gal( k/k). A homomorphism from W(k) to a discrete group is continuous iff the image of I is finite; such a homomorphism extends continuously to Gal( k/k) iff the image of all of W(k) is finite. To better understand the Weil group W(k) for non-archimedean k, we will now describe some subgroups of W(k) and the corresponding extension fields of k; these will be useful later on. First of all, a Galois extension L/k is unramified if a prime element in the integers of k remains prime in the integers of L. Equivalently, L/k is unramified if the inertia group I acts trivially on L. This implies that Gal(L/k) is generated by the image of F under the canonical map Gal( k/k) Gal(L/k). Since Z has a unique subgroup of every positive index, we see that for every d 1, the field k has a unique unramified extension k d k of degree deg(k d /k) = d. For example, the extension k 2 of k = Q p was described in section 3. The Weil group W(k d ) of k d is the subgroup of W(k) generated by I and F d. The composite field K := d 1k d is the maximal unramified extension of k in k; its Galois group is the inertia group of k: I = Gal( k/k). The residue field of K is f, and the natural map Gal(K/k) Gal( f/f) is an isomorphism. This means that the unramified extension k d /k can be constructed as k d = k(a), where a K is a lift of an element α f which generates the degree d extension f d /f. Thus, the theory of unramified extensions is more or less equivalent to the theory of extensions of finite fields. At the other extreme, a Galois extension L/k of degree e is totally ramified if a prime element in the ring of integers in k is the 21

22 e th power of a prime in L. Equivalently, L/k is totally ramified if the residue field of L is equal to the residue field of k. A general finite Galois extension L/k is of the form k k d L for some d, where L/k d is totally ramified of degree e = [L : k]/d. We say that L/k is tame if e is not divisible by the residue characteristic p of k. All of the L-packets constructed in this paper will arise from tame extensions. Totally ramified Galois extensions correspond to normal subgroups I of finite index, and the extension is tame if contains the maximal pro-p subgroup I + of I. The quotient I t := I/I + is called the tame inertia group ; it is the Galois group of the maximal tame extension K t of K. The field K t can be described very explicitly: it is obtained by adjoining to K all the roots of the polynomials X qn 1 +, for n 1, for any fixed choice of prime element in the ring of integers of k. This description of K t implies that the tame inertia group can be viewed as an inverse limit I t = lim f n, (25) where f n f is the extension of f of degree n and the transition maps are the norm homomorphisms f n f m, for m n. Under the isomorphism (25), the action of F by conjugation on I t corresponds to the automorphism α α q on each finite field f n. 6.2 Quasi-split groups and the L-group More details for this section can be found, for example, in [36]. An excellent survey on the origins of the L-group can be found in [7]. In this section G is a connected reductive algebraic group defined over the local field k. We also assume that the group G is quasi-split over k. This means that the flag variety B of G has a k-rational point, that is, G has a Borel subgroup B defined over k. (Note that G is no longer a compact real Lie group, as it has been until now.) Let T B be a maximal torus defined over k, contained in B, and let X = Hom(GL 1,T), Y = Hom(T,GL 1 ) 22

23 be the groups of algebraic co-characters (i.e., 1-parameter subgroups) and characters of T, respectively. Inside Y have the roots Φ of T in the Lie algebra of G, and the positive roots Φ + of T in the Lie algebra of B. Inside X we have the corresponding co-roots ˇΦ and positive co-roots ˇΦ +. The Galois group Gal( k/k) acts on X and Y, preserving Φ + and ˇΦ +. Let Gal( k/e) denote the kernel of this action; the field E is a finite Galois extension of k, called the splitting field of G. We say G is split if E = k. Thus, G determines a based root datum (X,Y, ˇΦ +, Φ + ) with an action of Gal(E/k). There is a unique (up to isomorphism) complex reductive Lie group Ĝ whose based root datum (Y,X, Φ +, ˇΦ + ) is dual to that of G. Moreover, there is a maximal torus and Borel subgroup ˆT ˆB in Ĝ such that X = Hom( ˆT,GL 1 ), Y = Hom(GL 1, ˆT), and ˇΦ + is the set of roots of ˆT in the Lie algebra of ˆB. The action of Gal(E/k) on this root datum extends to an action of Gal(E/k) by automorphisms of Ĝ, preserving ˆT and ˆB. The L-group of G is defined as the semi-direct product L G := Gal(E/k) Ĝ. Conversely, any continuous action of Gal( k/k) on a based root datum arises from a unique quasi-split group over k, up to isomorphism. Thus, the group G is determined by its L-group. The center Z( L G) of L G plays an important role, expecially in the p-adic case. One can show that Z( L G) = Z(Ĝ)Gal(E/k) is the fixed point group of Gal(E/k) in the center Z(Ĝ) of Ĝ. The group Z(L G) is finite precisely when G(k) has compact center. In particular, Z( L G) is finite if G is semisimple. We now consider some examples. If T is a torus over k, with character group Y, then T is quasi-split, with dual torus ˆT = Y C, and the L-group L T = Gal(E/k) ˆT is given by the action of Gal(E/k) on Y. 23

24 At the opposite extreme from tori, we have semisimple quasi-split groups. Here the Galois action on the root datum is given by an automorphism of the Dynkin diagram of G. (For the passage from root data to Dynkin diagrams, see [3, VI.4].) Let us consider the possibilities of this action for orthogonal groups, assuming that k C. If G = SO 2n+1 is a quasi-split odd orthogonal group, there are no nontrivial automorphisms of the Dynkin diagram, so the action of Gal( k/k) is trivial. Hence E = k and G is split. This means there is only one quasi-split odd orthogonal group, up to k-isomorphism. It arises from the quadratic form Q n + x 2 2n+1, where Q n = x 1 x n x n x 2n. In this case, we have L G = Ĝ = Sp 2n(C). If G = SO 2n is an even orthogonal group with n 2, then the Dynkin diagram has a symmetry of order two, so G is either split, or E is a quadratic field extension of k. These arise from the quadratic forms Q n, Q n 1 +N, respectively, where Q n is as above, and E is viewed as a two-dimensional k-vector space with quadratic form N : E k given by the norm. We call these two quasi-split groups SO 2n and SO 2n, respectively. (We omit the dependence on E since, in this paper, E/k will always be C/R or the unramified quadratic extension k 2 /k.) In the split case, we have L G = Ĝ = SO 2n(C). In the non-split case, we have L G = Gal(E/k) SO 2n (C), where the Galois action on SO 2n (C) is given by conjugation by a reflection in O 2n (C), and we have an isomorphism L G O 2n (C). In summary, we have the following table of L-groups for quasi-split orthogonal groups. G SO 2n+1 SO 2n SO 2n L G Sp 2n (C) SO 2n (C) O 2n (C) In all three cases we have Z( L G) = {±I} (except for SO 2, which is not semisimple). When k = R, these groups have real points SO 2n+1 (R) = SO(n + 1,n) SO 2n (R) = SO(n,n) SO 2n(R) = SO(n + 1,n 1), (26) 24

25 where SO(p,q) is the special orthogonal group of a quadratic form on R p+q of signature (p,q). 6.3 Langlands parameters Recall that G is a connected quasi-split reductive group over the local field k, whose splitting field E is a finite Galois extension of k. In the previous sections of this chapter we defined the Weil group W(k) of k, and the L-group L G = Gal(E/k) Ĝ. In this paper, a Langlands parameter has two ingredients. The first is a continuous homomorphism ϕ : W(k) L G, (27) whose projection onto the first factor Gal(E/k) is the composition of canonical maps W(k) Gal( k/k) Gal(E/k), (28) and whose projection to the second factor Ĝ consists of semisimple elements. For example, suppose E = k is p-adic. If ϕ is trivial on the inertia subgroup I of W(k), then ϕ is completely determined by the semisimple element ϕ(f) Ĝ. A more general Langlands parameter replaces the element ϕ(f) by an arithmetic refinement: the homomorphism ϕ : W(k) Ĝ. For another example, let T be a torus over k with character group Y = Hom(T,GL 1 ), with dual group ˆT = C Y, and L-group L T = Gal(E/k) ˆT. A Langlands parameter ϕ : W(k) L T amounts to a continuous one-cocycle on W(k) with values in ˆT, where W(k) acts on Ĝ via the natural map W(k) Gal(E/k). The second ingredient of a Langlands parameter is an irreducible representation of the finite group A ϕ = π 0 (CĜ(ϕ)) of connected components of the centralizer in Ĝ of ϕ. From basic homotopy theory we have a surjective homomorphism π 1 (Ĝ ϕ) A ϕ, where Ĝ ϕ Hom(W(k), L G) is the Ĝ-orbit of ϕ under conjugation. So a representation of A ϕ gives rise to a representation of the fundamental group of the Ĝ-conjugacy class of ϕ. In this paper, the complete Langlands parameter is a pair (ϕ,ρ), with ϕ as in (27) and ρ Irr(A ϕ ) is an irreducible representation of A ϕ. Two parameters 25

26 are considered equivalent if they are conjugate under Ĝ. Thus, the pair (ϕ,ρ) may be thought of as a conjugacy-class in Ĝ, with arithmetic and topological enhancements. Note that the center Z( L G) of L G is contained in CĜ(ϕ). We say that a Langlands parameter is discrete (some say elliptic) if the group CĜ(ϕ) has dimension as small as possible, that is, if CĜ(ϕ) and Z( L G) have the same identity component. For G semisimple, the Langlands parameter ϕ is discrete if and only if the group CĜ(ϕ) is finite, in which case we have A ϕ = CĜ(ϕ). A caveat: The Langlands parameters defined above are not sufficient to account for all representations in the p-adic case; one must also add a nilpotent part (see [37]). These more general parameters will play no role in this paper, but we will briefly mention them again in section 9, to give some perspective. 7 Real orthogonal groups again In this section we explicitly describe Langlands parameters for real orthogonal groups, and show how they relate to the L-packets contructed in section 4. Recall from (22) that the Weil group of R is W(R) = N H (C ). In particular, W(R) contains C as a normal subgroup of index two, with quotient Gal(C/R). It has a family of two-dimensional complex representations V (a), indexed by a 1 Z, defined by 2 V (a) = Ind W(R) C (χ a ), where χ a : C S 1 is the unitary character ( z z) a z 2a χ a (z) = = z z a. The representation V (a) is isomorphic to V ( a) = Ind(χ 1 a ), and is self-dual. It is symplectic when a 1 + Z and orthogonal when a Z. When V (a) is 2 orthogonal, its determinant is the sign character of W(R) ab = R. Finally, V (a) is irreducible unless a = 0, and V (0) is the direct sum of the trivial representation and the sign character. The Langlands parameters for SO 2n+1 over R are symplectic representations ϕ : W(R) L G = Sp 2n (C) = Sp(V ). 26

27 Discrete series parameters for SO 2n+1 are symplectic representations V of the form n V = V (χ i ), with i=1 χ i Z, χ 1 > χ 2 > > χ n > 0. Thus, ϕ corresponds to the positive character χ ρ + X(S C ) defined in section 5. We write Π(ϕ) = Π(χ). The compact torus S of section 4 is conjugate under SO 2n+1 (C) to the maximal compact subgroup of the complex torus T. Hence the involutions in S correspond to involutions in T. The latter involutions may be identified, by duality, with characters of the group ˆT 2 of involutions in ˆT. In turn, ˆT 2 is the the center of the subgroup Sp (V (χ 1 )) Sp (V (χ n )), and since the χ i are distinct, this center is exactly A ϕ = CĜ(ϕ). Thus, the L- packet Π(ϕ) is in bijection with the set of 2 n characters of A ϕ. For even orthogonal groups over R, recall from (26) that SO 2n denotes the quasi-split orthogonal group with SO 2n (R) = SO(n,n), and SO 2n denotes the quasi-split orthogonal group with SO 2n (R) = SO(n + 1,n 1). A Langlands parameter for either of these groups is an orthogonal representation ϕ : W(R) L G O 2n (C) = O(W) whose image is contained in SO 2n (C) when G = SO 2n and is not contained in SO 2n (C) when G = SO 2n. Discrete series parameters for real even orthogonal groups are orthogonal representations W of dimension 2n, of the form with W = n V (η i ), i=1 η i Z, η 1 > η 2 > > η n. Since each V (η i ) is orthogonal with nontrivial determinant equal to the sign character of W(R) ab = R, the resulting homomorphism ϕ : W(R) O(W) has image contained in SO(W) = SO 2n (C) precisely when n is even. Thus, ϕ is a discrete series parameter for SO 2n when n is even and SO 2n when n is odd. 27

28 In either case, ϕ corresponds to the positive character η ρ + X(S C ) defined in section 5. The involutions in S again correspond to characters of the centralizer A ϕ of ϕ in SO(W), and A ϕ is the center of the subgroup O (V (η 1 )) O (V (η n )). The 2 n characters of A ϕ parametrize the representations in the L-packet Π(ϕ) = Π(η) described in section 5. The above calculations may seem more natural when viewed in greater generality, and this will also permit later comparison with p-adic Langlands parameters. For any quasi-split real group G, a Langlands parameter ϕ : W(R) L G maps the normal subgroup C of W(R) to maximal torus in Ĝ, which, after conjugation, we may arrange to be ˆT. To be discrete, ϕ(c ) must contain a regular element of Ĝ. Then the centralizer of ϕ(c ) in Ĝ is exactly ˆT, and the full Weil group W(R) maps to the normalizer NL G( ˆT) of ˆT in L G. The element j, generating the quotient W(R)/C = Gal(C/R), maps to an element n NL G( ˆT), and conjugation by n is an involution of ˆT. The centralizer CĜ(ϕ) is the fixed points in ˆT of this involution. The discrete condition forces this involution to be inversion, and the centralizer A ϕ of ϕ is therefore always equal to the 2-torsion subgroup of ˆT. So the character group of A ϕ may be identified with the 2-torsion subgroup of a maximal compact torus in G(R), as in section 4. 8 Pure inner forms of p-adic groups The examples of L-packets for p-adic SO 3 in section 3 belong to a family of discrete series L-packets for p-adic groups whose Langlands parametrization (though not their construction) is similar to that for real groups. Later, we will discuss this family of p-adic L-packets in more detail. First, recall that in those examples we had two versions of SO 3, one compact and the other non-compact. In this section we explain how the various versions of a p-adic group are controlled by Galois cohomology sets H 1 (k,g). The real case also involved Galois cohomology, but in disguise. There, it sufficed to consider involutions s, which were the concrete manifestations of real Galois cohomology classes. Indeed, if G is a compact real group with maximal torus S and Weyl 28